In recent years, small electronic equipment represented by a mobile terminal is widely used, whereby further downsizing, reduction in the weight, and prolonged life are strongly requested. To meet the market needs as mentioned above, a secondary battery which can especially achieve downsizing, reduction in the weight, and high energy density is being developed. This secondary battery is being investigated for application not only to small electronic equipment but also to large electronic equipment represented by an automobile as well as a power storage system represented by a house.
Among them, a lithium ion secondary battery has high expectation because not only downsizing and increase in the storage capacity can be easily achieved but also a high energy density can be obtained as compared with a lead battery or a nickel cadmium battery.
The lithium ion secondary battery as mentioned above comprises a positive electrode, a negative electrode, a separator, and an electrolyte solution, wherein the negative electrode thereof includes a negative electrode active material which involves in the charge and discharge reactions.
In the negative electrode active material, carbon-based active materials are widely used, wherein a further increase in the battery capacity is requested from recent market needs. In order to increase the battery capacity, as the material for the negative electrode active material, use of silicon is being studied. This is because a significant increase in the battery capacity can be expected with silicon, for the theoretical capacity of silicon (4199 mAh/g) is more than 10 times than the theoretical capacity of graphite (372 mAh/g). Development of a silicon material as the material for the negative electrode active material are carried out with regard not only to a silicon simple substance but also to compounds represented by alloys, oxides or the like. Besides, with regard to the form of the active material in the carbon-based active material, the study is being made from a standard coating type to an integrated type directly deposited onto a current collector.
However, when silicon is used as a main raw material in the negative electrode active material, the negative electrode active material undergoes expansion and contraction during charge and discharge, so that a crack can easily occur mainly near to surface of the negative electrode active material. Furthermore an ionic substance is generated inside the active material, and the negative electrode active material becomes easily broken. When the surface of the negative electrode active material is cracked, a new surface is formed so that the reactive area of the active material increases. At this time, not only a decomposition reaction of the electrolyte solution takes place on the new surface but also the electrolyte solution is consumed because a film of the decomposition product of the electrolyte solution is formed on the new surface. Accordingly, the cycle property can be readily deteriorated.
Until now, in order to improve initial efficiency and cycle characteristics of a battery, various investigations have been made with regard to the electrode configuration as well as the negative electrode material for the lithium ion secondary battery mainly composed of a silicon material.
Specifically, in order to obtain excellent cycle characteristics and high safety, silicon and amorphous silicon oxide are simultaneously deposited by using a gas method (for example, see Patent literature 1). In order to obtain high battery capacity and safety, a carbon material (electronic conductive material) is disposed on surface of silicon oxide particles (for example, see Patent literature 2). In order to improve the cycle characteristics as well as to obtain the high input/output characteristics, an active material containing silicon and oxygen is produced, and also an active material layer having a high oxygen ratio near the current collector is formed (for example, see Patent literature 3). In order to improve the cycle characteristics, the silicon active material is made so as to contain oxygen with an average oxygen content of 40 at % or less and also to have a higher oxygen content near the current collector (for example, see Patent literature 4).
In order to improve a first time charge and discharge efficiency, a nano composite including an Si phase, SiO2, and MyO metal oxide is used (for example, see Patent literature 5). In order to improve the cycle characteristics, a mixture of SiOx (0.8≤x≤1.5 and particle diameter range of 1 to 50 μm) with a carbon material is burned at high temperature (for example, see Patent literature 6). In order to improve the cycle characteristics, a molar ratio of oxygen to silicon in the negative electrode active material is made 0.1 to 1.2, and the active material is controlled such that the difference between the maximum and minimum values of the molar ratio near the interface of the active material and the current collector is in the range of 0.4 or less (for example, see Patent literature 7). In order to improve the battery load characteristics, a metal oxide including lithium is used (for example, see Patent literature 8). In order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on the silicon material surface layer (for example, see Patent literature 9). In order to improve the cycle characteristics, silicon oxide is used, and on the surface layer thereof, a graphite film is formed so as to provide conductivity (for example, see Patent literature 10). In Patent literature 10, with regard to the shift values obtained from the Raman spectrum of the graphite film, broad peaks are appeared at 1330 cm−1 and 1580 cm−1 with the intensity ratio I1330/I1580 being 1.5<I1330/I1580<3. In order to obtain a high battery capacity and to improve the cycle characteristics, particles having a silicon microcrystal phase dispersed in silicon dioxide are used (for example, see Patent literature 11). In order to improve the over-charge and over-discharge characteristics, a silicon oxide in which atomic ratio of silicon to oxygen is controlled at 1:y (0<y<2) is used (for example, see Patent literature 12).